Theory of spin waves in short range order on the basis of the two dimensional t-J model

The Green's function theory of Kondo and Yamaji (KY theory) is extended to the t-J model on a square lattice. The KY theory describes spin wave excitations propagating in short range order and is a useful method to treat systems without symmetry breaking. It is found that the antiferromagnetic (AF) long-range-order (LRO) and correlation are sensitively destroyed by doping holes away from half-filling in the two dimensional case. The critical hole concentration for vanishing AF LRO is estimated to be much smaller than in the three dimensional case. (orig.)